The formation procedure of the LTPS TFT (Low Temperature Poly-silicon Thin Film Transistor) according to prior arts is: first deposing the buffer layer, and forming the semiconductor pattern layer. Furthermore, other elements of the LTPS TFT pixel unit, such as the insulative layer, the gate, etc. are formed on the semiconductor pattern layer.
Because the semiconductor pattern layer is formed on the buffer layer in the LTPS TFT pixel unit according to prior arts, that is to say, the semiconductor pattern layer is convex upward from the buffer layer, and the latter formed insulative layer forms stepped convex structures at two ends of the semiconductor pattern layer. Thus, the gaps of the gate and the semiconductor pattern layer formed on the buffer layer is not even to cause side effect. On the other hand, the stepped structures of the buffer layer exists, the signals becomes unstable in the transmission, and thus influences the electrical property of the LTPS TFT pixel unit.